Overview of Vitamins

Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe upper limit (tolerable upper intake level) has been determined for some vitamins. Intake above this limit increases the risk of a harmful effect (toxicity).

Consuming too little of a vitamin can cause a nutritional disorder. However, people who eat a variety of foods are unlikely to develop most vitamin deficiencies. Deficiency of vitamin D is an exception. Vitamin D deficiency Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, a deficiency can develop if people follow a restrictive diet that does not contain enough of a particular vitamin. For example, vegans, who consume no animal products, may become deficient in vitamin B12 Vitamin B12 Deficiency Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder. Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness... read more , which is available in animal products. Deficiency of the B vitamins biotin or pantothenic acid almost never occurs. People at high risk for vitamin deficiencies (for example, people who had bariatric surgery Bariatric Surgery Bariatric (weight-loss) surgery alters the stomach, intestine, or both to produce weight loss. In the United States, about 160,000 people have bariatric surgery each year. This number accounts... read more , are on hemodialysis Dialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more , or have alcohol use disorder Alcohol Use Alcohol (ethanol) is a depressant (it slows down brain and nervous system functioning). Consuming large amounts rapidly or regularly can cause health problems, including organ damage, coma,... read more ) may benefit from a daily multivitamin.

Consuming large amounts (megadoses) of certain vitamins (usually as supplements) without medical supervision may also have harmful effects.

Vitamins are called essential micronutrients because the body requires them but only in small amounts.

The body does not store most vitamins. Deficiencies of these vitamins usually develop in weeks to months. Therefore, people must consume them regularly.

Vitamins A, B12, and D are stored in significant amounts, mainly in the liver. Vitamins A and D are also stored in fat cells. Deficiencies of these vitamins take more than a year to develop.

Because many people eat irregularly or do not eat a variety of foods, they may not get enough of some vitamins from foods alone. If they do not get enough, the risk of certain cancers or other disorders may be increased. People may then take a multivitamin. However, for most people, taking multivitamins does not appear to reduce risk of developing cancer or heart or blood vessel (cardiovascular) disorders.

Some vitamins are fat soluble. Other vitamins are water soluble. The difference between fat and water soluble affects nutrition in several ways.